Employee Benefit Plans (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Regulatory Adjustment:
Amount of under-collection included in the two-way pension balancing account	$ (11,709,000)	$ (18,748,000)	$ (20,622,000)
Healthcare Cost Trend Rate
Initial health care cost for employees under age of 65 (as a percent)	8.00%	8.00%	8.00%
Ultimate health care cost for employees under age of 65 (as a percent)	7.20%	6.50%	6.00%
Period within which rate reaches ultimate trend rate	10 years	10 years	10 years
Initial Health care cost for employees of age 65 and over (as a percent)	7.00%	7.00%	7.00%
Ultimate health care cost for employees of age 65 and over (as a percent)	5.80%	5.80%	6.00%
Effects of one-percentage-point change in assumed health care cost trend rates
Effect on total of service and interest cost components -1 Percentage Point Increase	102,000
Effect on total of service and interest cost components -1 Percentage Point Decrease	(88,000)
Effect on post-retirement benefit obligation -1 Percentage Point Increase	1,464,000
Effect on post-retirement benefit obligation -1 Percentage Point Decrease	(1,263,000)
Defined contribution plan
Employer's contribution	1,900,000	1,900,000	1,800,000
Fixed income securities | Maximum
Employer's contribution and fair value of plan assets
Maturity period of investments	20 years
Fixed income securities | Minimum
Employer's contribution and fair value of plan assets
Maturity period of investments	1 year
GSWC
Regulatory Adjustment:
Amount of under-collection included in the two-way pension balancing account	(11,709,000)	(18,748,000)	(20,622,000)
Pension Benefits
Regulatory Adjustment:
Amount of under-collection included in the two-way pension balancing account	2,305,000	209,000	1,823,000
Actual and Target Asset Allocations
Actual Asset Allocations, Total (as a percent)	100.00%	100.00%
Target Asset Allocation for 2013, Total (as a percent)	100.00%
Employer's contribution and fair value of plan assets
Total investments measured at fair value	107,638,000	92,910,000	90,179,000
Employer contributions	6,117,000	6,563,000
Expected employer's contribution in 2013	6,600,000
Estimated future benefit payments
2013	4,336,000
2014	4,811,000
2015	5,325,000
2016	5,892,000
2017	6,503,000
Thereafter	42,491,000
Total	69,358,000
Expected long-term return on plan assets (as a percentage)	7.00%	7.00%	7.00%
Defined contribution plan
Employer's contribution	254,000	85,000
Pension Benefits | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	107,638,000	92,910,000
Pension Benefits | Equity securities
Actual and Target Asset Allocations
Actual Asset Allocations, Total (as a percent)	60.00%	56.00%
Target Asset Allocation for 2013, Total (as a percent)	60.00%
Employer's contribution and fair value of plan assets
Total investments measured at fair value	64,327,000	52,328,000
Pension Benefits | Equity securities | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	64,327,000	52,328,000
Pension Benefits | U.S. small cap stocks
Employer's contribution and fair value of plan assets
Total investments measured at fair value	5,396,000	4,288,000
Pension Benefits | U.S. small cap stocks | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	5,396,000	4,288,000
Pension Benefits | U.S. mid cap stocks
Employer's contribution and fair value of plan assets
Total investments measured at fair value	10,722,000	8,616,000
Pension Benefits | U.S. mid cap stocks | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	10,722,000	8,616,000
Pension Benefits | U.S. large cap stocks
Employer's contribution and fair value of plan assets
Total investments measured at fair value	31,966,000	26,803,000
Pension Benefits | U.S. large cap stocks | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	31,966,000	26,803,000
Pension Benefits | International funds
Employer's contribution and fair value of plan assets
Total investments measured at fair value	10,797,000	7,997,000
Pension Benefits | International funds | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	10,797,000	7,997,000
Pension Benefits | Real estate funds
Employer's contribution and fair value of plan assets
Total investments measured at fair value	5,446,000	4,624,000
Pension Benefits | Real estate funds | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	5,446,000	4,624,000
Pension Benefits | Debt securities
Actual and Target Asset Allocations
Actual Asset Allocations, Total (as a percent)	39.00%	42.00%
Target Asset Allocation for 2013, Total (as a percent)	40.00%
Pension Benefits | Cash equivalents
Actual and Target Asset Allocations
Actual Asset Allocations, Total (as a percent)	1.00%	2.00%
Employer's contribution and fair value of plan assets
Total investments measured at fair value	1,721,000	1,682,000
Pension Benefits | Cash equivalents | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	1,721,000	1,682,000
Pension Benefits | Fixed income securities
Employer's contribution and fair value of plan assets
Total investments measured at fair value	41,590,000	38,900,000
Pension Benefits | Fixed income securities | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	41,590,000	38,900,000
Pension Benefits | GSWC
Regulatory Adjustment:
Number of water regions for which two-way balancing account was established	3
Amount of under-collection included in the two-way pension balancing account	(2,300,000)	(209,000)	(1,800,000)
Post-Retirement Medical Plans
Actual and Target Asset Allocations
Actual Asset Allocations, Total (as a percent)	100.00%	100.00%
Target Asset Allocation for 2013, Total (as a percent)	100.00%
Employer's contribution and fair value of plan assets
Total investments measured at fair value	8,359,000	7,916,000	6,959,000
Employer contributions	155,000	951,000
Expected employer's contribution in 2013	150,000
Estimated future benefit payments
2013	477,000
2014	509,000
2015	578,000
2016	652,000
2017	728,000
Thereafter	4,690,000
Total	7,634,000
Post-Retirement Medical Plans | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	8,359,000	7,916,000
Post-Retirement Medical Plans | Equity securities
Actual and Target Asset Allocations
Actual Asset Allocations, Total (as a percent)	61.00%	60.00%
Target Asset Allocation for 2013, Total (as a percent)	60.00%
Post-Retirement Medical Plans | U.S. large cap stocks
Employer's contribution and fair value of plan assets
Total investments measured at fair value	5,102,000	4,752,000
Post-Retirement Medical Plans | U.S. large cap stocks | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	5,102,000	4,752,000
Post-Retirement Medical Plans | Debt securities
Actual and Target Asset Allocations
Actual Asset Allocations, Total (as a percent)	38.00%	39.00%
Target Asset Allocation for 2013, Total (as a percent)	40.00%
Post-Retirement Medical Plans | Cash equivalents
Actual and Target Asset Allocations
Actual Asset Allocations, Total (as a percent)	1.00%	1.00%
Employer's contribution and fair value of plan assets
Total investments measured at fair value	65,000	100,000
Post-Retirement Medical Plans | Cash equivalents | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	65,000	100,000
Post-Retirement Medical Plans | Fixed income securities
Employer's contribution and fair value of plan assets
Total investments measured at fair value	3,192,000	3,064,000
Post-Retirement Medical Plans | Fixed income securities | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	3,192,000	3,064,000
Union plan
Estimated future benefit payments
Expected long-term return on plan assets (as a percentage)	7.00%	7.00%	7.00%
Non-union plan
Estimated future benefit payments
Expected long-term return on plan assets (as a percentage)	4.20%	4.20%	4.20%
SERP
Estimated future benefit payments
2013	318,000
2014	317,000
2015	316,000
2016	337,000
2017	406,000
Thereafter	3,581,000
Total	5,275,000
Balance in Rabbi Trust
Balance in Rabbi Trust	4,800,000
SERP | Rabbi Trust
Employer's contribution and fair value of plan assets
Total investments measured at fair value	4,816,000	2,829,000
SERP | Rabbi Trust | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	4,816,000	2,829,000
SERP | Rabbi Trust | Equity securities
Employer's contribution and fair value of plan assets
Total investments measured at fair value	2,822,000	1,739,000
SERP | Rabbi Trust | Equity securities | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	2,822,000	1,739,000
SERP | Rabbi Trust | Cash equivalents
Employer's contribution and fair value of plan assets
Total investments measured at fair value	116,000	9,000
SERP | Rabbi Trust | Cash equivalents | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	116,000	9,000
SERP | Rabbi Trust | Fixed income securities
Employer's contribution and fair value of plan assets
Total investments measured at fair value	1,878,000	1,081,000
SERP | Rabbi Trust | Fixed income securities | Level 1
Employer's contribution and fair value of plan assets
Total investments measured at fair value	$ 1,878,000	$ 1,081,000